The present invention relates to a system for remote sensing of the features of a celestial body, and, more particularly, to such a system with a star-sensing attitude determination subsystem.
Remote sensing of celestial bodies, including the earth, is finding increasing applications in meteorology, land resources surveying, planet mapping, and surveillance. In the future, remote sensing will play an increasingly important role as the human race explores and monitors planets, comets, asteroids and other natural and man-made celestial bodies. Remote sensing systems typically employ a satellite, or other spacecraft, with one or more image sensors, and at least one remote station for image display.
Generally, remote sensing involves mapping individual picture elements (pixels) to a coordinate system representing the scene being surveyed. For example, in locating a particular weather feature, a meteorological satellite system assigns to each pixel an earth latitude and longitude. This mapping consists of two transformations: an attitude transformation relating pixel and inertial coordinates, and an orbital transformation relating inertial and earth coordinates.
In order for the mapping to be valid, both transformations must be performed accurately. The accuracy of the transformation to earth coordinates depends on the accuracy of attitude and location of the camera or other imaging device. Errors in attitude and location can result in the mislocation of weather features, for example. Where multiple sensors are involved, e.g. sensors incorporating detectors with different spatial or spectral sensitivities, the mappings for separate sensors should be coaligned to provide a consistent composite image. Alternatively, a way of co-registering the data from multiple sensors is often required.
Since a small angular error translates into a significant displacement on the surface of a scene when spacecraft distances are involved, attitude determination is particularly critical. For example, at geosynchronous altitude, about 35,800 km, generally used for meteorological applications, a 280 .mu.RAD angular displacement results in a 10 km ground displacement. In certain of the ground operations, for example, gridding and wind velocity extraction, such errors can seriously affect forecasting accuracy so that it is necessary to keep these errors as small as possible.
The gridding requirements vary by application. NASA/GSFC (National Aeronautics and Space Administration/Goddard Space Flight Center) has indicated a registration requirement of 2 km earth surface gridding accuracy and the National Weather Service requires 1 km accuracy.
Users have found the NOAA/NESDIS (National Oceanic and Atmospheric Administration/National Earth Satellite Data and Information Service) grids can sometimes be incorrect by as much as 50 km. Also, spacecraft maneuver performance uncertainties, usually greater than 10%, result in inaccuracies in post-maneuver predicted orbit-attitude states. Accordingly, for about eight hours following a spacecraft orbit maneuver, the grid placement accuracy is sometimes worse than 50 km.
Thus, it has been very difficult to obtain the precision required for detailed mapping or coalignment. For example, spacecraft attitude uncertainties currently limit the precision of earth-referenced grid determination and placement on meteorological satellite images, as well as the accuracy of products such as wind vectors. Hence, the precision of subsequent meteorological forecasts is degraded in time and/or location.
Many satellites have incorporated sensor subsystems dedicated to attitude determination. For example, the orientation state of a spinning spacecraft is usually determined by dedicated earth, sun, and/or star sensors.
However, there is a problem coaligning the dedicated attitude sensor with the primary imaging payload. Pre-launch calibration is suspect in view of the stresses of satellite deployment which can alter the relative physical location of the attitude and imaging sensors. In addition, on-orbit thermal distortion can significantly alter the relative sensor alignment as a function of time.
Since the star, sun and earth-limb sensing attitude determination subsystems have incorporated sensors in addition to the primary surveying sensor, there has been a penalty in cost and complexity. For example, a typical star sensor can cost $1,000,000, weigh 50 lbs., and consume 50 watts. Two or three are normally required for accuracy as well as redundancy since satellites are relatively inaccessible for repairs. The additional power, weight and complexity interact more than linearly to increase satellite system design, manufacture and launch costs.
Attempts have been made to employ a primary imaging sensor for attitude determination to meet the stringent accuracy requirements of geosynchronous meteorological satellites. To date, two such methods have been used operationally: earth-landmark tracking and earth-horizon or earth-limb tracking.
As indicated by the performance of current weather satellites, earth features are generally not sufficiently distinct to provide precise attitude determinations, and they are subject to cloud obscuration. Furthermore, where the same features are used to determine satellite location as well as attitude, uncertainties are introduced in attempting to decouple the two variables. In addition, decoupling burdens the computational system with complex algorithms, consuming time and costly processing power.
Occasional pointing of a sensor toward the sun has been considered. However, it has proved difficult to manufacture high resolution sun sensors, due in part to "jitter", e.g., electronic sensor or solar variations. Furthermore, the sun provides only one inertial reference and at least two inertial references are required to determine attitude.
Star sensors have an advantage over earth and sun sensors in that their targets are point source emitters of predictable amplitude. However, operational star-sensing systems utilize a separate attitude sensor so coordination between the star (or separate sun sensor) data and the surveying sensor introduces relative alignment uncertainties as a function of time. This is due in part to initial misalignment, mechanical distortions due to launching stresses, and thermal distortions as a function of time in orbit.
Bright stars are occasionally observed within the operational field of view of a primary surveying sensor just above the limb of the earth, as noted by Doolittle et al. in "Attitude Determination Support for SMS/GOES Satellites", NOAA Technical Memorandum NESS 64, C. L. Bristor, editor, 1975, pp. 26-32. However, since the probability that a star of useful magnitude will appear above the earth's limb in, for example, a 20.degree..times.20.degree. frame is about 2%, it has not been feasible to use only such star information routinely in attitude determination.
A surveying earth sensor located on the rotor of a spin-stabilized spacecraft has provided attitude data from stars during special backscan tests, as disclosed by McIntyre et al. in "A Star Scan/Attitude Determination Experiment Conducted on the Geostationary Meteorological Satellite", Acta Astronautica, Vol. 7, pp. 137-154, Pergamon Press Ltd., 1980, U.K. The authors were able to detect four or five stars generally visible and available for attitude determination. They also disclosed algorithms for determining spacecraft attitude from the meteorological satellite star data.
McIntyre et al. acknowledge that their attitude determination procedure interfered unacceptably with the primary meteorological viewing of the sensor. This is due in part to the time involved in gathering the required volume of star data. In the disclosed system, star sensing precluded earth observation for more than an hour. This preclusion translated into the loss of multiple frames of earth meteorological data. This magnitude of loss is unacceptable as a normal operating mode.